1. Field of the Invention
This invention relates to a method for treatment of cyanide-containing waste water. More particularly, the present invention relates to treatment of waste water which is exhausted from chemical industries involving surface treatments of metals and metal refining, dyeing, etc.
2. Description of the Prior Art
In metal plating industries, waste waters containing dissolved cyanides of metals such as Zn, Cu, Ni, Fe, etc., are generally unavoidably produced in a dilute or condensed form. These cyanide-containing waste waters contain a cyano component in an extremely high concentration which in a condensed waste water amounts to as high as 30,000 - 50,000 ppm. Although these waste waters have considerably great toxicity, satisfactory methods for treating such waste waters in a technically and economically satisfactory manner have not been known in the art. This leaves the serious problem of environmental pollution unsolved.
As a matter of fact, a few methods have thus far been proposed and reduced to practice for rendering the condensed waste waters harmless in individual waste water-exhausting plants, including an oxidizing method using chlorine and an electrolytic oxidation method. However, these methods are disadvantageously complicated in treatment operations and have the possibility of generating toxic gases such as hydrogen cyanide, cyanogen chloride, etc. Additionally, the sludge resulting from these methods contains various kinds of metals but is discharged as it is since there have been no methods developed to treat the sludge in an effective manner.
On the other hand, it has been proposed to treat at one time waste waters collected from various plating and metal refining plants in a common treating system by acid decomposition, boiling and condensing or by ion-exchange. However, such methods disadvantageously give rise to various problems to be solved including generation of a large amount of hydrogen cyanide gas, formation of sludge which contains non-decomposed cyanides in an amount as high as several tens percent, and the necessity for safety measures for the apparatus which is used to carry out the particular method.
Moreover, there is also well known a method wherein cyanide-containing waste water is boiled down and condensed, or decomposed by the action of an acid to generate hydrogen cyanide which is charged in a subsequent stage into a burning furnace for combustion. However, it is essentially required in this method to treat the waste water in two distinct stages, i.e., complete decomposition of cyano components into hydrogen cyanide and separate combustion of the resultant cyanide, and to use a large-sized apparatus to conduct such two-stage treatment.
More recent methods directed to the treatment of cyanide-containing waste waters include the method disclosed in U.S. Pat. No. 3,215,524 wherein an aqueous formaldehyde solution is added to cyanide-containing waste water for liberating metals contained in the water, and then the metals are separated by adsorption using an ion-exchange resin, the method disclosed in U.S. Pat. No. 3,505,217 wherein cyanide-containing waste water is first mixed with an aldehyde or a mixture of formaldehyde bisulfite, and then further mixed with a mineral acid for decomposition, and the method disclosed in D.T. No. 2,119,119 wherein cyanides of metals which are contained in waste water are subjected to reaction with formaldehyde for recovering the metals in the form of alkali metal salts of glycolic acid. However, the solution which is obtained after decomposition and/or treatment with an ion-exchange resin for adsorbing and separating metals still disadvantageously has a high Chemical Oxygen Demand value and accordingly cannot be discharged from the treatment plant as it is. In addition, the metals contained in waste waters cannot be also advantageously collected in the form of useful compounds. Thus, these prior art methods cannot be adopted as a treating method of cyanide-containing waste water without entailing the disadvantages described above.